Before the Child Citizenship Act took effect in Feb. 2001, adoption by US citizens didn't automatically confer citizenship; the adoptive parents had to explicitly apply for their children's citizenship.
Your parents didn't do that (unless they did it and didn't tell you?), and you turned 18 before Feb 27, 2001 so you are not a US citizen and not eligible to file N-600. To become a citizen you will have to apply for citizenship using the usual N-400 process.
PD: Jan 2003 (EB3 rest of world)
I-485 Approved: July 2007
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.